Title pretty much says it all. A lot of TV shows on TheTVDB don't have episode thumbs. I would say that the frame grab should be configurable to avoid getting the same thumb for every episode should it fall within the opening credits.
As an extension to this feature, possibly even the ability to upload those thumbs to TheTVDB (I don't know how impossible this would be, so I wouldn't consider it a must)
Thanks again for a great app. Looking forward to the next version.